Salinas, Calif. ? California produce growers and processors worked to draw up new food safety measures Thursday as government investigators trying to pinpoint the source of the deadly E. coli outbreak narrowed their search to three counties.

Trade groups hoped to deliver improved guidelines to the Food and Drug Administration within a week but were unsure how long it would take to win the agency’s approval.

“We have people who hope this will be resolved soon so they can salvage something of this season,” said Tom Massif, president of Western Growers, an industry group representing about 3,000 fruit and vegetable farmers in California and other states. “Once we go to Washington and iron out those guidelines, we’ll be much closer to a date.”

Federal officials have set the new food-safety measures as a prerequisite for lifting a week-old consumer warning on fresh spinach.

Massif said it was too early to provide details, but that the measures likely would focus on better water and soil testing and beefed-up sanitation standards for field workers and packaging plants.